182: Identification and characterization of interferon-λ4 (IFN-λ4), a novel class-2 cytokine which impairs clearance of hepatitis C virus

We have identified a novel human interferon, designated as interferon-λ4 (IFN-λ4) (Prokunina-Olsson et al., Nature Genetics, 2013). Inducible expression of IFNL4 , along with three related genes ( IFNL1 , IFNL2 and IFNL3 ) located on chromosome 19q13.13 and encoding other interferon-λ proteins, IFN-λ1, IFN-λ2 and IFN-λ3, was revealed by RNA-sequencing in primary human hepatocytes treated with polyI:C. In humans, IFN-λ4 is fully genetically controlled – it is produced only by mRNA transcripts with a frameshift dG allele of a genetic variant ss469415590 (TT/dG) within the first exon of the gene, while transcripts with the TT allele can generate only unrelated proteins or protein fragments. We characterized IFN-λ4 as a class-2 cytokine based on its protein homology with IFN-λ3 (29% amino acid identity), the ability to cause STAT1 and STAT2 phosphorylation, activation of an ISRE-Luc reporter, induction of ISGs and antiviral response in HepG2 cells transiently transfected with an IFNL4-expressing construct. IFN-λ4 signaling is decreased by a JAK inhibitor, siRNA-silencing of IFN-λR1, and blocking of IL10R2. However, HepG2 cells did not respond to treatment with purified recombinant IFN-λ4 protein. Protein expression of IFN-λ4 is detectable in cells transfected with an IFNL4-expressing construct but not in the culture supernatants. It remains unclear whether IFN-λ4 is an unusual intracellular interferon, which signals through yet unknown intracellular partners or it is released at low levels through secretion or other mechanisms. Individuals homozygous for the derived human-specific ss469415590-TT allele (90% of Asians, 50% of Europeans and 10% of individuals of African ancestry) are genetically unable to produce IFN-λ4. This pattern suggests strong positive selection for elimination of IFN-λ4, which might be caused by current or historic infectious diseases. Individuals who are unable to produce IFN-λ4 are more likely to clear hepatitis C virus (HCV) either spontaneously or after treatment with IFN-α and ribavirin.